Node.js功能流程

Question

When I get a request, I want it to generate a 4-character code, then check if it already exists in the database. If it does, then generate a new code. If not, add it and move on. This is what I have so far:

var code = "";
var codeFree = false;
while (! codeFree) {
    var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    var code = "";
    for (var i = 0; i < 4; i++) {
        var rand = Math.floor(Math.random() * chars.length);
        console.log(rand);
        code += chars.charAt(rand);
    }
    console.log("Code: %s generated.", code);

    client.execute("select * from codes where code=" + code, function(err, result) {
        if (! err) {
            if (result.rows.length > 0) {
                codeFree = false;
            } else {
                codeFree = true;
            }
        } else {
            console.log('DB ERR: %s', err);
        }
        console.log(codeFree);
    });
    console.log('here');
}

This does not do nearly what I want it to do. How can I handle something like this?

Answer 1

You are doing an async task.

When you have an asyncronous task inside your procedure, you need to have a callback function which is going to be called with the desired value as its argument.

When you found the free code, you call the function and passing the code as its argument, otherwise, you call the getFreeCode function again and passing the same callback to it. Although you might consider cases when an error happens. If your the db call fails, your callback would never get called. It is better to use a throw / catch mechanism or passing another argument for error to your callback.

You can achieve what you need to do by doing it this way:

function getFreeCode(callback) {
    var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    var code = "";
    for (var i = 0; i < 4; i++) {
      var rand = Math.floor(Math.random() * chars.length);
      console.log(rand);
      code += chars.charAt(rand);
    }
    console.log("Code: %s generated.", code);

    client.execute("select * from codes where code="+code, function(err, result) {
      if(!err) {
        if(result.rows.length > 0) {
          getFreeCode(callback);
        } else {
          callback(code);
        }
      }else {
        console.log('DB ERR: %s', err);
      }
      console.log(codeFree);
    });
    console.log('here');
}




// in your main:

getFreeCode(function (code) {
    console.log(' this code was free: ' + code)
})

Answer 2

I recommend you look into two alternatives to help deal with asynchronous code.

1. node generator functions using the 'yield' keyword
2. promises

Using generators requires running a recent version of node with the --harmony flag. The reason I recommend generators is because you can write code that flows the way you expect.

var x = yield asyncFunction();
console.log('x = ' + x);

The previous code will get the value of x before logging x.

Without yielding the console.log would write out x before the async function was finished getting the value for x.

Your code could look like this with generators:

var client = {
    execute: function (query) {
        var timesRan = 0;
        var result = [];
        return function () {
            return setTimeout(function () {
                result = ++timesRan < 4 ? ['length_will_be_1'] : [];
                return result;
            },1);
        };
    }
};

function* checkCode () {
    var code;
    var codeFree = false;
    while(!codeFree) {
    var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    code = "";
    for (var i = 0; i < 4; i++) {
      var rand = Math.floor(Math.random() * chars.length);
      console.log(rand);
      code += chars.charAt(rand);
    }
    console.log("Code: %s generated.", code);

    try {
        var result = yield client.execute("select * from codes where code="+code);
      codeFree = result.rows.length > 0 ? false : true;
      }catch(e) {
        console.log('DB ERR: %s', err);
      } finally {
        console.log(codeFree);
      }

    console.log('here');
  }
}

checkCode().next();

You would leave off the client object. I only added that to make a working example that fakes an async call.

If you have to use an older version of node or do not like the yield syntax then promises could be a worthy option.

There are many promise libraries. The reason I recommend promises is that you can write code that flows the way you expect:

asyncGetX()
.then(function (x) {
    console.log('x: ' + x);
});

The previous code will get the value of x before logging x.

It also lets you chain async functions and runs them in order:

asyncFunction1()
.then(function (result) {
    return asyncFunction2(result)
})
.then(function (x) { /* <-- x is the return value from asyncFunction2 which used the result value of asyncFunction1 */
    console.log('x: ' + x);
});

Your code could look like this with the 'q' promise library:

var Q = require('q');

var client = {
    timesRan: 0,
    execute: function (query, callback) {
        var self = this;
        var result = {};
        setTimeout(function () {
            console.log('self.timesRan: ' + self.timesRan);
                result.rows = ++self.timesRan < 4 ? ['length = 1'] : [];
                callback(null, result);
        },1);
    }
};

function checkCode () {
    var deferred = Q.defer();
    var codeFree = false;
  var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
  var code = "";
  for (var i = 0; i < 4; i++) {
    var rand = Math.floor(Math.random() * chars.length);
    console.log('rand: %s', rand);
    code += chars.charAt(rand);
  }
  console.log("Code: %s generated.", code);

    client.execute("select * from codes where code="+code, function(err, result) {
        console.log('err: '+err+', result: ' + JSON.stringify(result));
        console.log('result.rows.length: ' + result.rows.length);
    if(!err) {
      if(result.rows.length > 0) {
        codeFree = false;
        console.log('result.rows: %s, codeFree: %s', result.rows, codeFree);
        checkCode();
      } else {
        codeFree = true;
        console.log('line 36: codeFree: ' + codeFree);
        deferred.resolve(code);
      }
    }else {
      console.log('DB ERR: %s', err);
          deferred.reject(err);
    }
    console.log(codeFree);
  });
  console.log('waiting for promise');

  return deferred.promise;
}

checkCode()
.then(function (code) {
    console.log('success with code: ' + code);
})
.fail(function(err) {
  console.log('failure, err: ' + err);
});

Also omit the client object here. I only added that to make a working example that fakes an async call.

Promises and generators definitely take some time to get used to. It's worth it because they make the code a lot easier to follow in the end than code written with nested callbacks.